This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-361228, filed Dec. 20, 1999; No. 2000-236324, filed Aug. 4, 2000; No. 2000-366040, filed Nov. 30, 2000; and No. 2000-377867, filed Dec. 12, 2000, the entire contents of all of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus including: a paper carrying unit which carries paper; a paper support unit which has a paper support surface and supports the paper being carried by the paper carrying unit by the paper support surface; and an image forming unit which is arranged on the opposite side of the paper support surface of the paper support unit with respect to the paper and forms an image on the paper being carried by the paper carrying unit.
2. Description of the Related Art
In an image forming apparatus, an image forming unit forms a desired image on a sheet of paper supported on a paper support surface of a paper support unit. In order to improve the quality of an image to be formed, a distance between the paper supported on the paper support surface of the paper support unit and the image forming unit must be maintained constant (for example, 1 to 2 mm, or more preferably within 1 mm).
In particularly, in an ink-jet printer which is one type of the image forming apparatus and uses an ink-jet head as the image forming unit, the paper must be prevented from being lifted above the power support surface in addition to maintaining the distance between the paper and the ink-jet head constant. That is because the lifted paper is brought into contact with the ink-jet head if the paper is lifted above the paper support surface, which results in a factor of paper jam or a damage to an ink nozzle of the ink-jet head.
Jpn. Pat. Appln. KOKAI Publication No. 8-156351 and Jpn. Pat. Appln. KOKAI Publication No. 9-220837 disclose an ink-jet printer in which a plurality of elongated openings or a plurality of grooves which extend in the paper carrying direction are formed on a paper support surface of a platen. Further, these publications disclose that an air suction unit sucks the paper to the paper support surface through the plurality of elongated openings or a plurality of grooves and the paper is prevented from being lifted above the paper support surface.
In the paper support unit disclosed in the above two publications, however, the large suction force can not be caused to act on the paper until the paper closes all of the plurality of openings or the plurality of grooves. In order to prevent the paper from being lifted from the power support surface, a quantity of air suction of the air suction unit must be increased, thereby deteriorating the energy efficiency. Furthermore, when a quantity of air suction of the air suction unit is too large, the resistance between the paper support surface and the paper becomes large, and a quantity of carrying the paper by the paper carrying unit may become inappropriate or paper jam may occur.
In view of the above-described drawbacks, it is a first object of the present invention to provide an image forming apparatus which prevents a sheet of paper from being lifted above a paper support surface of a paper support unit. It is another object of the present invention to provide an image forming apparatus having the excellent energy efficiency of a paper carrying unit.
In the field of the image forming apparatus, improvement in an image forming speed is a permanent problem. One solution of this problem in the ink-jet printer is extension of a length of the ink-jet head in the paper carrying direction. Moreover, with lengthening of the ink-jet printer, a length of a platen in the paper carrying direction must be also increased.
When a long platen is used, however, the possibility that the paper is lifted from a recording range on the paper support surface is increased.
As factors of occurrence of this problem, the following can be considered.
(1) The remaining stress generated when uniformly cutting the paper to a predetermined width dimension in a manufacturing process, is actualized at both ends of the paper in the widthwise direction, and the paper extends along the paper carrying direction.
In addition, the paper extends along the paper carrying direction due to, e.g., an impact of moisture absorption. At this moment, extension in the paper carrying direction generated at the both ends of the paper in the widthwise direction is larger than extension in the paper carrying direction generated at the central part of the paper in the widthwise direction. As a result, the wavelike swell which makes progress in the paper carrying direction is generated at the both ends of the paper in the widthwise direction. This is a factor of occurrence of lift of the paper at the both ends in the paper widthwise direction.
(2) When the length of the platen in the paper carrying direction of the paper support surface is long and an area of the paper support surface becomes large, a supported area of the paper supported by the paper support surface is enlarged. As a result, paper lift generated at the both ends of the paper in the widthwise direction becomes very large. It is hard to cause the lifted part of the paper to be appressed against the paper support surface by only the suction force of the air suction unit. Additionally, since the length of the paper support surface in the paper carrying direction is long, it is also difficult to move the lifted part of the paper to any position other than the recording range on the paper support surface.
(3) As to the paper lift generated at the both ends of the paper in the widthwise direction, since a convex portion of the wavelike swell extends along the paper widthwise direction (see FIG. 10A and FIG. 10B), the paper lift can not be effectively sucked by the ink-jet printer disclosed in the above two publications.
That is, in the platen disclosed in the above two publications, since the plurality of openings or the plurality of groove are formed along the paper carrying direction, the paper lift that the convex portion of the wavelike swell is generated in the paper carrying direction can be effectively sucked, but the paper lift that the convex portion of the wavelike swell is generated along the paper widthwise direction can not be effectively sucked.
In view of the above-described problems, it is a second object of the present invention to provide an image forming apparatus which prevents both ends of paper from being lifted above a paper support surface of a paper support unit even if wavelike swell is generated along the paper carrying direction at the both ends of the paper in the widthwise direction.
To achieve the first object, according to the present invention, there is provided a first image forming apparatus including a paper support unit which has a paper support surface supporting paper, the paper support unit comprising:
a plurality of grooves formed along the paper carrying direction; and
a plurality of suction holes respectively formed to each of the plurality of grooves,
wherein positions of the plurality of suction holes in two adjacent grooves in the plurality of grooves are staggered in the paper carrying direction.
An end of the paper carried on the paper support surface covers the suction holes of one groove in the adjacent grooves, and the paper is thereby sucked to the paper support surface. Further, when the paper is carried, the end of that paper covers the suction holes of the other groove, and the papery is thereby sucked to the paper support surface. That is, before the paper reaches the lower end of the paper support surface, the paper is sucked alternately by the plurality of suction holes arranged in the adjacent groove so as to be staggered in the vertical direction, thereby preventing the paper from being lifted.
Furthermore, in order to achieve the first object, according to the present invention, there is provided a second image forming apparatus, wherein a chamber constituting a suction unit which sucks the paper to the paper support surface is divided into a plurality of parts in the paper carrying direction.
The end of the paper carried on the paper support surface is first sucked on the paper support surface by the chamber on the upstream side. Then, when the paper is further carried, the end of the paper is sucked on the paper support surface by the chamber on the downstream side. That is, with increase in an area of the paper supported on the paper support surface, a chamber to be used can be changed. Therefore, the paper can be prevented from being lifted above the paper support surface, and the energy efficiency of paper carriage can be improved.
Moreover, in order to achieve the first object, according to the present invention, there is provided a third image forming apparatus, wherein there is provided a control unit which controls a suction negative pressure generated by the suction unit which sucks the paper onto the paper support surface.
Therefore, the paper can be efficiently sucked on the paper support surface without causing the paper to be lifted above the paper support surface.
In addition, in order to achieve the second object, according to the present invention, there is provided a fourth image forming apparatus, wherein the paper support unit has a first paper support surface portion which supports both ends of the paper in the widthwise direction and a second paper support portion which supports a central part of the paper in the widthwise direction, and
wherein a percentage of a length L1 of the first paper support surface portion in the paper carrying direction relative to a length L0 of the paper support unit in the paper carrying direction is set larger than a percentage of a length L2 of the second paper support surface portion in the paper carrying direction relative to the length L0 of the paper support unit in the paper carrying direction.
A cause of generation of the paper lift is a fact that the length in the paper carrying direction at the both ends of the paper in the widthwise direction and in the vicinity of the both ends is longer than the length in the paper carrying direction at the central part of the paper in the widthwise direction and the paper support surface of the paper support unit is flat irrespective of generation of the wavelike swell which makes progress along the paper carrying direction at the both ends of the paper in the widthwise direction.
Accordingly, as described above, the paper lift can be prevented by extending the length in the paper carrying direction by forming, e.g., a plurality of convex portions on the first paper support surface portion, and by sucking the extension generated at the end of the paper in the widthwise direction, i.e., the wavelike swell into the convex portions.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.